1. Field of the Invention
The present invention relates to a dielectric filter having an auxiliary conductor of a predetermined pattern which is provided on an open end surface and is electrically connected to an outer conductor provided on a side surface. The present invention also relates to a method of manufacturing such a dielectric filter.
2. Description of the Related Art
There are known various kinds of dielectric filters in which the inner surfaces of through-holes extending through a dielectric ceramic block are coated with conductive material forming inner conductors so as to produce a plurality of resonators disposed in parallel, and in which the outer surface of the dielectric ceramic block is coated with a conductive material forming an outer conductor, except for an open end surface at which one end of each of the through-holes opens.
Further, a dielectric filter having an improved structure is disclosed in Japanese Utility Model Publication (kokoku) No. 4-8643. As shown in FIG. 1 which is based on the disclosure of that publication, in the improved dielectric filter, strip-shaped or strip-form auxiliary conductors f, i.e., conductor strips, are formed on the open end surface d such that one auxiliary conductor f is located between resonators a and b, the other auxiliary conductor f is located between resonators b and c, and the auxiliary conductors f are electrically connected to an outer conductor e formed on the outer surface. Due to formation of the strip-form conductors f, the frequency characteristic of the resultant dielectric filter exhibits an attenuation peak on the higher frequency side with respect to the center frequency. Through modification and adjustment of the shape and the width of the auxiliary conductors f, the mutual electromagnetic coupling between the resonators a and b and between the resonators b and c can be adjusted.
In addition, as shown in FIG. 2, in the dielectric filter disclosed in Japanese Patent Application Laid-Open (kokai) No. 9-219605, a counterbore h is formed at the end of a through-hole g that opens at the open end surface d, such that the diameter of the counterbore h increases at the open end; and an inner conductor j covering the inner surface of the counterbore h is extended inwardly in the radial direction, to thereby increase the effective resonance length. An advantage of this structure is that the overall length of the dielectric filter is decreased as compared with a conventional dielectric filter having the same resonance length.
It will be appreciated that, depending on the purpose or use to which a dielectric filter is to be put, there is a demand for various different types of dielectric filters having different shapes and characteristics. In this regard, there is a demand for a dielectric filter which exhibits a filter characteristic having an attenuation peak on the higher frequency side thereof and which can be made compact. Such a dielectric filter can be produced by combining the filter structure shown in FIG. 1 in which the strip-form conductors f are formed such that the filter characteristic has a particular attenuation peak, and the filter structure shown in FIG. 2 in which the overall length is shortened through formation of the counterbore h. However, in this case, because both the strip-shaped or strip-form conductors f and the counterbore h must be formed on the same open end surface, the simultaneous formation of the strip-shaped conductors f and the counterbore h on this surface is difficult because the surface is limited in area.
Moreover, the basic dielectric filter shown in FIG. 1 has a particular drawback. Specifically, the auxiliary conductors f and the outer conductor e are formed through a process in which a conductive material in the form of paste is applied, by means of screen printing, to each surface of the dielectric ceramic block in a predetermined pattern, followed by a baking step carried out at a predetermined temperature. In order to provide the required connections between the auxiliary conductors f on the open end surface d and the outer conductor e on the outer surface, the conductive paste material must be applied such that a conductive material layer on the open end surface and the conductive material layer on a selected side surface must overlap each other at an edge portion where the open end surface and the selected side surface intersect each other, so as to ensure that a mutual connection is established between the conductive material layers. However, since each layer of applied conductive material is very thin and becomes thinner at the edge portion due to surface tension of the conductive material, when the baking step is carried out, the thin portion of the conductive material layer located at the edge portion of the block can easily break due to the difference in the thermal expansion coefficients between the dielectric ceramic block and the conductive material, with the result that such electrode breakage can occur at the edge portion.
The above-described drawback of the layer of applied conductive material, i.e., the fact that the layer becomes thinner at the edge portion due to surface tension, can be overcome by manually applying the conductive material to the edge portion after completion of screen printing to thereby increase the thickness of the layer to a desired level. Since a manual operation is difficult and time consuming, and manufacturing efficiency is low, with the result that the abovementioned solution is simply not practical.
Moreover, in the conventional dielectric filter, when the auxiliary conductors f are formed, a conductive material in the form of paste is screen-printed on the flat open end surface. Therefore, the conductive material easily runs and spreads, which makes accurate formation of the auxiliary conductor f impossible as a practical matter.
In view of the foregoing, a first object of the present invention is to provide a dielectric filter which has a shortened overall length and which can be made to exhibit a peak in the filter characteristic thereof on the higher-frequency side with respect to the center frequency.
Another object of the present invention is to provide a dielectric filter which has a structure that prevents electrode breakage which would otherwise occur at an edge portion, as described above, and which also enables the accurate formation of auxiliary conductors in a desired pattern.
Still another object of the present invention is to provide a method of manufacturing a dielectric filter, which method prevents electrode breakage which would otherwise occur at an edge portion and enables the accurate formation of auxiliary conductors in a desired pattern.
According to a first aspect of the invention, there is provided a dielectric filter including at least three resonators, the filter comprising: a dielectric ceramic block having at least three through-holes formed therein, in parallel, in a group having opposite ends, each of the through-holes including an inner surface, and the at least three through-holes including end through-holes located at opposite ends of the group and at least one intermediate through-hole located between the end through-holes, the dielectric ceramic block including an open end surface at which one end of each of said through-holes opens; an inner conductor covering the inner surface of each of the through-holes to respectively form corresponding end resonators and at least one intermediate resonator; an outer conductor covering a predetermined outer surface of the dielectric ceramic block, excluding said open end surface; and input/output pads formed on one side surface of the dielectric ceramic block such that the pads are located near the open end surface and face the end resonators so as to be capacatively coupled thereto, each of the end through-holes including a counterbore formed at one end of the corresponding end through-hole and opening at the open end surface such that the corresponding end through-hole is of increased diameter at said one end; and the filter further comprising an auxiliary conductor disposed on the open end surface so as to face said at least one intermediate resonator, the auxiliary conductor being of forked shape and having at least two branch portions, each of the branch portions being located between adjacent resonators, and the auxiliary conductor at least partially surrounding the at least one intermediate resonator with an insulating gap formed therebetween, and the auxiliary conductor being connected to a portion of the outer conductor located on the one side surface on which the input/output pads are formed.
In this aspect of the invention, each of the end through-holes includes a counterbore at one end of the corresponding through-hole which opens at the open end surface and thus the corresponding through-hole is of an increased diameter at the one end. As a consequence, the inner conductor covering the inner surface of the counterbore extends inwardly in the radial direction, so that the effective resonance length is increased. Thus, the lengths of the end resonators can be shortened and still provide the same resonance characteristics.
Further, because for at least one resonator other than the end resonators, an auxiliary conductor connected to the outer conductor on the side surface is disposed such that the auxiliary conductor at least partially surrounds the at least one resonator with an insulating gap formed therebetween, a capacitance (C) is produced between the auxiliary conductor and the inner conductor of the at least one resonator, and as will be described later, the length of the at least one resonator can be shortened by an amount related to the value of the capacitance C. Accordingly, the resonance length of the at least one resonator can be made to be the same as that of the end resonators by adjustment of the capacitance C to a desired value through adjustment of the insulating gap and the length of the arcuate edge portion of the auxiliary conductor surrounding the at least one resonator. Thus, the overall length of the dielectric filter can be correspondingly shortened. Further, the resultant dielectric filter has a frequency characteristic which exhibits an attenuation peak on the higher-frequency side of the center frequency.
Preferably, the auxiliary conductor has an open configuration including an opening which opens toward the side surface opposite to that on which the input/output pads are formed; and a second auxiliary conductor extends toward the opening from a portion of the outer conductor located on that opposite side surface, such that the inner edge of the second auxiliary conductor faces the corresponding resonator with an insulating gap formed therebetween. In this case, an additional capacitance is produced between the second auxiliary conductor and the inner conductor of the corresponding resonator, so that the overall capacitance C can be increased.
In one embodiment of the dielectric filter according to this first aspect of the present invention, a trap resonator is disposed at one end of the dielectric ceramic block such that the trap resonator is located adjacent to at least one of the end resonators. The inner surface of the through-hole of the trap resonator is covered with an inner conductor, and a counterbore is formed at one end of the through-hole which opens at the open end surface so that the diameter of the through-hole is increased at the one end. In this case, a trap effect is produced at a self-resonance frequency outside the pass band, whereby spurious signals can be attenuated to provide elimination thereof.
According to a second aspect of the invention, there is provided a dielectric filter including a plurality of resonators, the filter comprising: a dielectric ceramic block including a plurality of through-holes formed therein in parallel, said through-holes including an inner surface and the dielectric ceramic block having an open end surface at which one end of each of said through-holes opens; an inner conductor covering the inner surface of each of the through-holes to thereby form a corresponding resonator; an outer conductor covering a predetermined outer surface of the dielectric ceramic block, excluding the open end surface; and an auxiliary conductor formed on the open end surface in a predetermined pattern, the auxiliary conductor being electrically connected to a portion of the outer conductor located on one side surface of the dielectric ceramic block, the auxiliary conductor comprising a conductive material disposed in a concave portion formed in the open end surface of said block in a pattern corresponding to the pattern of the auxiliary conductor.
In this aspect of the invention, because the shape of the conductive material is defined by the concave portion, i.e., flow of the conductive material is restricted by the concave portion, the conductive material neither runs nor spreads. Thus, the auxiliary conductor can be accurately formed in a desired pattern.
According to the second aspect of the present invention, there is further provided a method of manufacturing a dielectric filter, the method comprising the steps of: press forming ceramic powder to produce a green body having a shape of a substantially rectangular prism and including a concave portion of a predetermined shape formed in an end surface of the prism; sintering the green body to obtain a dielectric ceramic block; disposing a conductive material paste form in the concave portion; applying a conductive material onto a predetermined outer surface of the dielectric ceramic block excluding said end surface; and baking said conductive material to form an outer conductor on the predetermined outer surface of the dielectric ceramic block excluding said end surface so that said end surface serves as an open end surface of the filter and to form an auxiliary conductor in the concave portion of the end surface such that an outer end of the auxiliary conductor is electrically connected to a portion of the outer conductor located on one side surface of the dielectric ceramic block.
When this method is used, a concave portion of predetermined shape can readily be formed in the green body simultaneously with formation of the green body, by using a mold which has on the inner surface thereof a projecting portion corresponding to the concave portion.
In the method set forth above, when a conductive paste material is used to fill the concave portion formed on the open end surface to a thickness corresponding to the depth of the concave portion, the outer end surface of the layer of the conductive material is made to be flush with the corresponding side surface of the dielectric ceramic block. When the conductive paste material is then applied onto the outer surface of the dielectric ceramic block, a layer of the conductive material formed on the side surface of the dielectric ceramic block, and serving as the outer conductor, is connected to the outer end surface of the layer of the conductive material disposed in the concave portion. Thus, the edge portion at which the bottom surface of the concave portion intersects with the side surface of the dielectric ceramic block is covered with a thick layer of the conductive material. Accordingly, the layers at the edge portion will each attain a desired level of strength during a subsequent baking step. Thus, electrode breakage can be prevented which would otherwise occur at the edge portion due to a difference in the thermal expansion coefficients of the dielectric ceramic block and the conductive material.
According to this second aspect of the present invention, there is further provided a dielectric filter having three or more resonators, the filter comprising: a dielectric ceramic block having at least three through-holes formed therein, in parallel, in a group having opposite ends, the at least three through-holes including end through-holes located at opposite ends of the group and at least one intermediate through-hole located between the end through-holes, each of the through-holes having an inner surface and the dielectric ceramic block having an open end surface at which one end of each of the through-holes opens; an inner conductor covering the inner surface of each of the through-holes to respectively form corresponding end resonators and at least one intermediate resonator; an outer conductor covering a predetermined outer surface of the dielectric ceramic block, excluding the open end surface; and input/output pads formed on one side surface of the dielectric ceramic block such that the pads are located near the open end surface and face said end resonators so as to be capacatively coupled thereto, each of the through-holes of the end resonators including a counterbore formed at one end of the corresponding through-hole and opening at the open end surface such that the corresponding through-hole is of increased diameter at said one end; and the filter further comprising an auxiliary conductor disposed on the open end surface so as to face the at least one intermediate resonator, the auxiliary conductor being of a forked shape pattern including at least two branch portions, each of said branch portions being located between adjacent resonators, and the auxiliary conductor at least partially surrounding the at least one intermediate resonator with an insulating gap formed therebetween and being connected to a portion of the outer conductor located on said one side surface on which the input/output pads are formed; and the auxiliary conductor being formed by a conductive material disposed in a concave portion formed in the open end surface in a pattern corresponding to the pattern of the auxiliary conductor.
In this filter construction, as in the case of the dielectric filter according to the first aspect of the invention, the overall length of the dielectric filter can be shortened, and the dielectric filter has a frequency characteristic exhibiting an attenuation peak on the higher-frequency side thereof.
In addition, because the shape of the conductive material is defined by the concave portion, i.e., flow of the conductive material is restricted by the concave portion, the conductive material neither runs nor spreads over the surface of the block. Moreover, since the auxiliary conductor can be formed accurately such that the insulating gap has an intended size and the arcuate edge portion of the auxiliary conductor surrounding the at least one resonator has an intended length, a desired capacitance C can be reliably obtained. Furthermore, since the auxiliary conductor has a thickness corresponding to the depth of the concave portion, the end surface of the auxiliary conductor facing the at least one resonator has a larger area, so that a larger capacitance C can be obtained.
The dielectric filter can be manufactured by the method described above. In this case, the edge portion at which the bottom surface of the concave portion intersects with the corresponding side surface of the dielectric ceramic block is covered with a thick layer of the conductive material. Accordingly, the layers at the edge portion will each attain a desired strength during a subsequent baking step. Thus, electrode breakage can be prevented which would otherwise occur at the edge portion due to a difference in thermal expansion coefficients between the dielectric ceramic block and the conductive material.
Preferably, the auxiliary conductor has an open configuration including an opening which opens toward the side surface opposite to that on which the input/output pads are formed; a second auxiliary conductor extends toward the opening from a portion of the outer conductor located on the opposite side surface, such that the inner edge of the second auxiliary conductor faces the corresponding resonator with an insulating gap formed therebetween; and the second auxiliary conductor is formed by conductive material disposed in a second concave portion formed in the open end surface and having a pattern corresponding to the pattern of the second auxiliary conductor. In this case, an additional capacitance is produced between the second auxiliary conductor and the inner conductor of the corresponding resonator, so that the overall capacitance C is increased. Moreover, the second auxiliary conductor can be accurately formed so that the insulating gap is of an intended size and the second auxiliary conductor is of an intended shape. Furthermore, since the end surface of the second auxiliary conductor facing the corresponding resonator is larger in area, a larger capacitance C can be obtained. In addition, electrode breakage does not occur at the edge portion.